The Islamic State is a nasty problem that can be remedied if its neighbors, assisted by the United States, decide to do so. Vladimir Putin's fascist revival is a crisis that tests the West's capacity to decide.

Putin's serial amputations of portions of Ukraine, which began with his fait accompli in Crimea, will proceed, and succeed, until his appetite is satiated. Then the real danger will begin.

Suppose Ukraine is merely his overture for the destruction of NATO, the nemesis of his Soviet memory. Then what might be his version of the Gleiwitz radio station episode 75 years ago?

On the evening of Aug. 31, 1939, Nazi SS personnel pretending to be Polish partisans seized the station, which was about four miles inside Germany (Gliwice is now in Poland), proclaiming that Poland was invading Germany to achieve "our just [territorial] claims," and shot a German prisoner dressed in a stolen Polish uniform, giving Hitler his pretext for declaring war the next day.

Putin has discarded the minor inhibitions of what NATO calls his "hybrid war"  giving slightly surreptitious aid to Russian separatists; brazenly infiltrating Russian soldiers in unmarked uniforms. Russia has invaded Ukraine, although the Obama administration likes the semantic anesthesia of calling it an "incursion." Putin does not pretend that it will be, like President Nixon's 1970 "incursion" into Cambodia, temporary.
So, suppose Putin, reprising his Ukrainian success, orchestrates unrest among the Russian-speaking minorities in Latvia, Lithuania or Estonia. Then, recycling Hitler's words that his country "could not remain inactive," Putin invades one of these NATO members. Either NATO invokes Article 5 an attack on any member is an attack on all  or NATO disappears and the Soviet Union, NATO's original raison d'etre, is avenged.

Although no one more thoroughly detested the regime that Gen. Erwin Rommel served, Winston Churchill acknowledged in January 1942 in the House of Commons the talent of Britain's enemy: "We have a very daring and skillful opponent against us and, may I say across the havoc of war, a great general." Putin is, the West should similarly acknowledge, more talented and dangerous than either Nikita Khrushchev or Leonid Brezhnev.

Their truculence was not fueled by fury. Putin's essence is anger. It is a smoldering amalgam of resentment (of Russia's diminishment because of the Soviet Union's collapse), revanchist ambitions (regarding formerly Soviet territories and spheres of influence), cultural loathing (for the pluralism of open societies) and ethnic chauvinism that presages "ethnic cleansing" of non-Russians from portions of Putin's expanding Russia.

This is more than merely the fascist mind; its ethnic-cum-racial component makes it Hitlerian. Hence Putin is "unpredictable" only to those unfamiliar with the 1930s. Regarding the roles of resentment and vengeance, remember where Hitler insisted that France formally capitulate in 1940  in the railroad carriage, near the town of Compiegne, where Germany signed the 1918 armistice.

Since its emancipation by the Soviet Union's demise, Ukraine has been ravaged by corruption that frays national sentiment, which even before this was a tenuous phenomenon. In "The Long Shadow: The Legacies of the Great War in the Twentieth Century," David Reynolds of Cambridge University cites a British diplomat's 1918 analysis:

"Were one to ask the average peasant in the Ukraine his nationality, he would answer that he is Greek Orthodox; if pressed to say whether he is a Great Russian, a Pole, or an Ukrainian he would probably reply that he is a peasant; and if one insisted on knowing what language he spoke, he would say that he talked 'the local tongue.'"

Ukraine may be an ethnic casserole susceptible to diminishment by Putin's ladle. But the Baltic states, by virtue of their NATO membership, are, regardless of their histories or sociologies, decisively different. And given Putin's animus, nourished by his negligibly resisted success in Ukraine, he is more dangerous than the Islamic State.

This group is perhaps 20,000 fighters possessing some artillery and armor but no air force. It is an island of tenuously occupied territory in a sea of hostile regimes  those of Saudi Arabia, Jordan, Syria, Iran, Iraq and Iraq's Kurdish region, which has its own regime. These command approximately 2 million troops who, with ample air power, can pulverize the Islamic State whenever the regimes summon the will to do so.

U.S. participation in this should be conditional on the regional powers putting their militaries where their mouths (sometimes) are in the fight against radical Islamists. U.S. participation in defense of the Baltic states is unconditional.